Hey, and welcome to Focus Fanatics! Ignore the bickering, it doesn't happen often around here lol.
As far as wheel size goes, I think most folks will agree that 17's are a pretty good balance. They fill up a lot of the ugly wheelwell gap, and yet they aren't so big that you have to worry about rubbing unless you get a crazy suspension drop on there. Also, they won't make the ride super harsh. One of the tradeoffs with an increased wheel diameter is increased ride harshness. So, just something to think about there!
Personally, I think the new foci can pull of 18's a lot better than the older models, but I'd probably still go with 17's.
ROTM June '13, PNW Crew #42, SR #245, Rally Rev #02